1. Field of the Invention
The present invention relates to an output driver, and more particularly, to a differential output driver and a semiconductor device having the same, in which a differential input signal is received and a differential output signal is generated.
2. Description of the Related Art
A differential output driver of a conventional semiconductor device receives a differential input signal and generates an output signal and an inverted output signal which have a voltage difference within a constant range. However, values of elements constituting the differential output driver vary due to changes in process, voltage, or temperature, resulting in the output signal and the inverted output signal having a voltage difference outside of a desired range.
FIG. 1 is a circuit diagram of an example of the conventional differential output driver, which includes PMOS transistors P1 and P2, termination resistors RT1 and RT2, NMOS transistors N1 and N2, and a bias current source Ibias.
The circuit shown in FIG. 1 operates as follows:
When an on-die termination control signal ODTEN of “low” level is applied, the PMOS transistors P1 and P2 are turned on so that the termination resistors RT1 and RT2 are connected between a power supply voltage VDD and differential output nodes n1 and n2. In this case, when differential input signals DIN and DINB of “high” level and “low” level are applied, the NMOS transistor N1 is turned on and the NMOS transistor N2 is turned off. Accordingly, an inverted output signal DQB of “low” level and an output signal DQ of “high” level are generated. In contrast, when differential input signals DIN and DINB of “low” level and “high” level, respectively, are applied, the NMOS transistor N1 is turned off and the NMOS transistor N2 is turned on. Accordingly, an inverted output signal DQB of “high” level and an output signal DQ of “low” level are generated. The termination resistors RT1 and RT2 serve to prevent the differential output signals DQ and DQB from being reflected when the differential output driver outputs the differential output signals DQ and DQB.
When the on-die termination control signal ODTEN of “high” level is applied, all of the PMOS transistors P1 and P2 are turned off, so that the termination resistors RT1 and RT2 are not connected to the power supply voltage and the differential output driver is disabled.
However, in the differential output driver shown in FIG. 1, values of the termination resistors RT1 and RT2 and other elements may vary due to changes in process, voltage, or temperature, or a swing width of the output signal DQ may differ from a swing width of the inverted output signal DQB due to a mismatch in load on a signal line transmitting the pair of differential output signals DQ and DQB. Consequently, the signal width of the pair of different output signals DQ and DQB becomes narrower, which degrades signal characteristics.